Flextensional vibration sensor

ABSTRACT

A vibration sensing device, package and system and a method of manufacturing a vibration sensing device are disclosed. The vibration sensing device comprises a body whose geometry is such that it exhibits flextensional properties.

The present invention relates to the field of vibration sensing and inparticular, in certain preferred embodiments to acceleration sensing orseismic sensing.

Current vibration sensing devices and, in particular, seismic sensors,such as geophones or accelerometers, typically use electronictransducers such as transducers using the reverse piezo-electric effect.Such devices generally use a mass-spring transducer mounted in a casing,the casing being in intimate contact with the surrounding medium inwhich the vibrations are propagating. These devices require on-boardelectronic conditioning circuitry. These electronics not only need powerwhich may have to be transmitted for long distances they also requirepackaging to preserve them from a highly corrosive, vibrational and highpressure environment. The packaging and power requirements all lead tobulky installations. Furthermore, the electronics must also be highlyreliable so that retrieval of the sensor package is not required; thismeans that the electronics package will be correspondingly expensive.

Generally, in the seismic field a plurality of seismic sensing devicesare used together in an array and it is these arrays that form thebulkiest equipment in seismic sensing. Reducing the size of theindividual sensing devices is therefore very important in this area.

A further problem with conventional electronic accelerometers that usethe reverse piezo electric effect in high temperature seismic:applications is that they experience reduced sensitivity when the Curietemperature is approached. This reduction in sensitivity can, in extremecases, lead to a nullification of the piezo electric effect; this is anon-reversible process.

Alternatives to piezo-electric vibration sensing devices include fibreoptic vibration sensing devices, these have the advantages of beinglight, being linked by optical fibre rather than copper cables and ofusing a fibre optic interferometric CONFIRMATION COPY measurementtechnique which is a highly sensitive technique. Furthermore, they areresistant to temperatures up to several hundred degrees centigrade andare therefore unaffected by temperatures they are likely to meet in downhole oil field measurements. These devices are arranged such the opticalfibre of the interferometer is stressed by some means. Generally, thisis done by mounting the optical fibre on some flexible mandrel or discstructure. Unfortunately, these devices suffer from being relativelyinsensitive for their size as the fibre is stressed via a second ordereffect. In order to achieve high sensitivity the devices tend to requirea large mass, typically greater than 500 grams. This not only makes thedevice bulky and heavy it also limits the frequency range in which thesensor can be used. An example of a flexural disc fibre opticaccelerometers are given below.

U.S. Pat. No. 5,317,929 discloses a fibre optic accelerometer. Thisaccelerometer comprises a pair of flexible disks mountedcircumferentially on a rigid cylindrical body and having a mass mountedon a rod which extends between the disks and is centrally attached toboth. An optical fibre is mounted in a spiral on the lower side of theupper disk and the upper side of the lower disk. The optical fibres areaccessible at one end, having reflective portions at the other. Anyvariation in the length of the spiral optical fibres caused by flexureof the disks is detected and provides an indication of accelerationcaused by physical displacement, shock or vibrations.

Vibration sensing devices such as accelerometers and in particular,those used in seismic sensing typically aim to measure signals such asacceleration in one direction and to be resistant to signals in otherdirections. The devices of the prior art suffer from problems ofcross-axis sensitivity with off axis acceleration signals affecting thedetected results of these accelerometers. Furthermore, many of theseismic sensing devices of the prior art are insensitive and thus, donot detect small vibrations. In an attempt to increase sensitivity someof the devices incorporate large masses, these tend to make the deviceheavy and unwieldy. It is desirable to produce a vibration sensor havinglow threshold levels in the on axis orientation and low responsivity inthe off axis direction.

According to a first aspect of the invention there is provided avibration sensing device comprising: a hollow flextensional body havinga cross section that has a major and a minor axis, said flextensionalbody being operable to deform in response to received vibrational energyand thereby produce a variation in a predetermined property exhibited bysaid flextensional body in the direction of one of said major or minoraxis, said flextensional body being shaped such that said variation insaid predetermined property is amplified in the direction of the otherof said major or minor axis; and a sensor coupled with the flextensionalbody and operable to detect the amplified variation indicative ofreceived vibrational energy.

The use of a flextensional body to receive vibrational energy provides asystem in which a variation in a property caused by vibration receivedby the flextensional body can be amplified by the flextensional body.This amplified property is sensed by the sensor. The properties of theflextensional body are such that comparatively small bodies and massescan be used, making it convenient to use in an array. Thus, a sensitiveand convenient sensing device can be produced. Although acoustictransmitters are known which employ the flextensional principle togenerate sonar waves. These transmitters are large, heavy devices beingof the order of one metre high, one metre long and half a metre wide,they also carry heavy metal struts to protect them. When placed in waterthey are often powerful enough to make the water boil. The skilledperson looking for a small, yet sensitive vibrational detector would nothave thought of looking at these devices.

In preferred embodiments, said predetermined property comprises force,vibrational energy received “along said minor axis causing deformationof said flextensional body, with a force acting along said minor axisand an amplified force acting along said major axis, said sensor beingcoupled to said flextensional body along said major axis to detect theamplified force.

This arrangement providing amplification of the vibration force that isto be detected at the sensor has been found to increase the sensitivityof the sensor significantly.

In other embodiments said predetermined property comprises displacement,vibrational energy received along said major axis causing displacementof said flextensional body along said major axis and an amplifieddisplacement along said minor axis, said sensor being coupled to saidflextensional body along said minor axis to detect the amplifieddisplacement.

Although an outer housing is not necessary preferably said vibrationsensing device further comprises an outer housing, said vibrationsensing device being mounted within said outer housing. An outer housingprotects the device and can provide a suspension system which reducescross axis sensitivity. The outer housing can be sealed and filled witha damping fluid or it may be left open to the external environment.

Preferably, said vibration sensing device further comprises a massmounted on said flextensional body. A mass increases the sensitivity ofthe device, the vibrational force causing acceleration of the mass.

In some embodiments, said minor axis has a first and a second end,wherein said sensor is mounted along said major axis of saidflextensional body and said mass is mounted to said flextensional bodyin the proximity of said first end of said minor axis. This arrangementprovides increased sensitivity due to the mass and amplifcation of thevibrational force due to the arrangement of the sensor, thus aparticularly sensitive device is achieved.

With this arrangement it is preferable to mount said flextensional bodyto said outer housing via mounting means, said mounting means connectinga portion of said flextensional body in the proximity of said second endof said minor axis to said housing.

In other embodiments said major axis has a first and a second end,wherein said sensor is coupled along said minor axis of saidflextensional body and said mass is mounted to said flextensional bodyin the proximity of said first end of said major axis. This arrangementprovides amplification of the displacement which particularly with amass attached provides a sensitive device.

With this arrangement it has been found to be preferable to mount saidflextensional body on said outer housing via mounting means, saidmounting means connecting a portion of said flextensional body in theproximity of said second end of said major axis to said housing.

Although the flextensional body can be formed of any material exhibitingappropriate properties and in particular stiffness, preferably, saidflextensional body is formed of metal. Metal is a robust material thatis sensitive to vibrational energy and exhibits suitable properties fortransmitting vibrational energy to a sensor mounted upon it.

Preferably, said flextensional body comprises a tube having anelliptical cross section. An elliptical shape has been found to beparticularly effective as a vibration sensor. It is a convenient shapefor mounting the sensor on, and amplification of a received signal isboth pronounced and mathematically easy to predict. It has also beenfound that a vibration sensor having an elliptical geometry can be madeconveniently small and yet still be sensitive to vibration across arange of frequencies.

Although the mass may be mounted on the outside surface of theflextensional body, preferably, it is mounted within said flextensionalbody. This arrangement provides a compact device. This is an advantagein many applications such as in seismic sensing where a large number ofthese devices will be used together in an array.

Advantageously, said flextensional body comprises an outer wall, saidouter wall having a substantially uniform thickness. Flextensionalproperties of a body depend to some extent on the wall thickness of thebody being substantially constant.

In preferred embodiments, said major axis of said flextensional body isbetween 10 mm and 30 mm, preferably, 22 mm and said minor axis of saidflextensional body is between 5 mm and 20 mm preferably 11 mm.

The dimensions of the flextensional sensors are small making the devicesboth small and light and suitable for mounting in an array. This is verydifferent to flextensional transmitters which are large, heavy, unwieldydevices that are generally used singly.

Although the vibration sensing devices can have a number of differentforms in some embodiments said vibration sensing device comprises anaccelerometer.

In some embodiments said sensor comprises a strain sensor.

In preferred embodiments said sensor comprises an optical fibre coupledto said flextensional body such that deformation of said flextensionalbody produces a strain in said optical fibre which imposes a variationin at least one predetermined property of an optical signal transmittedthrough said optical fibre, said optical fibre being arranged such thatat least one end is accessible for optical coupling to an optical devicecomprising a detector for detecting said changes in said at least onepredetermined property of said transmitted optical signal.

Optical fibre sensors are extremely sensitive. Furthermore, they aresmall and light and do not require electrical power. Additionally, thefibre can be used as both the means to sense the signal and to transmitthe data back to the interrogation unit. This means that there is noneed to house any electronics in the deployed part of any sensing array.This leads to significant reductions in the size, complexity and hencecost of the system. Optical fibre sensors are also generally resistantto high temperatures and corrosive environments and as such areparticularly well adapted for use in seismic applications.

Preferably, said optical fibre is coupled under stress to saidflextensional body. By pre-stressing the optical fibre the net forceacting on the fibre will generally remain either compressive orextensional. This helps keep the sensor in the linear displacementregime and avoids non-linearities in the detection region.

In some embodiments, said vibration sensing device further comprisesblocks mounted on the outer surface of said flextensional body at eitherend of said minor axis, said optical fibre sensor comprising an opticalfibre coil, said optical fibre coil being coupled to said flextensionalbody by being wound around said blocks, whereas in others said vibrationsensing device further comprises blocks mounted on the outer surface ofsaid flextensional body at either end of said major axis, said opticalfibre sensor being coupled to said flextensional body by being woundaround said blocks.

The blocks provide a convenient surface for the optical fibre to bemounted upon. Furthermore, their properties can be chosen to provide asurface that is not rough or sharp and thus will not damage the fibre.Additionally they provide an increased radius for the fibre to bewrapped around, this increases the length of sensing fibre that can beused thereby increasing its sensitivity. The blocks also increase theradius of the curve around which the fibre is wrapped, this decreaseslight loss which may occur if the bend in the curve is too sharp.Furthermore, wrapping an optical fibre directly on the flextensionalbody may inhibit its flexing properties.

Preferably, said blocks are formed of metal. Metal is a robusttemperature resistant material. Furthermore, the material properties ofmetal are such that the physical response of the device will berelatively unaffected by the changes in temperature which may beencountered in some applications such as seismic applications. It isgenerally found to be appropriate to have blocks made from the samematerial as the flextensional body.

Advantageously, said optical fibre is coupled to said flextensional bodysuch that both ends of said optical fibre are accessible for opticalcoupling to further optical devices. Many devices of the prior art onlyhave one end of any optical fibre sensor accessible for coupling toexternal devices, the other end being located well within the device andhaving a reflective portion. Thus, any signal travelling through thefibre from the accessible end, is reflected back through the fibre bythe reflective portion, exiting the fibre at the end that it entered. Inthe device according to an embodiment of our invention, the arrangementof the device is such that both ends of the optical fibre are accessiblefor external connection. Thus, a signal entering the device via one endof the fibre can exit it via the other end; this makes the deviceparticularly adaptable for configuration into an array architecture.Optical fibre strain sensors are also particularly suitable for use inan array architecture with a high degree of multiplexing using bothDense Wavelength Division Multiplexing (DWDM) and Time DivisionMultiplexing (TDM) being possible with these sensors.

A second aspect of the present invention comprises a vibration sensingpackage, comprising three vibration sensing devices according to a firstaspect of the present invention, wherein each of said three vibrationsensing devices have a sensor coupled along an axis of said sensingdevice, said three vibration sensing devices being mounted such thatsaid axes along which respective sensors are coupled are arrangedorthogonally to one another.

Each of said individual vibration sensing devices is sensitive tovibrations along a particular axis. Mounting three such sensing devicesorthogonally to each other provides a sensing package that is sensitiveto vibrations in any direction. Such a package can detect vibrations inthree dimensions and provide a directional component to the vibrationsthat are sensed.

In preferred embodiments said package further comprising a hydrophone. Ahydrophone within the package provides a package that can measure boththe pressure and the shear wave in a vibration sensing system.

A third aspect of the present invention provides a vibration sensingsystem, comprising: a first plurality of vibration sensing devicesaccording to a first aspect of the present invention; an electromagneticradiation source and an electromagnetic radiation detector; said opticalfibres of said first plurality of vibration sensing devices beingarranged in optical communication with each other and with saidelectromagnetic radiation source and detector; said electromagneticradiation source being operable to transmit an optical signal into saidoptical fibres of said plurality of vibration sensing devices; and saidelectromagnetic radiation detector being arranged to receiveelectromagnetic radiation output from said plurality of vibrationsensing devices and to detect a variation in at least one predeterminedproperty of said output optical signal.

The vibration sensing devices of the present invention being small andlight and having both ends of the optical fibre available areparticularly well adapted to mounting in an array in a vibration sensingsystem. This makes them particularly suitable for use as seismic sensingdevices.

In some embodiments the vibration sensing devices are arranged opticallyin series, whereas in others they may be arranged in parallel or in bothparallel and series.

A further aspect of the present invention provides, a method ofdetecting vibrations comprising: coupling a sensor to a hollowflextensional body having a cross section that has a major and a minoraxis, said flextensional body being operable to deform in response toreceived vibrational energy and thereby produce a variation in apredetermined property exhibited by said flextensional body in thedirection of one of said major or minor axis, said flextensional bodybeing shaped such that said variation in a predetermined property isamplified in the direction of the other of said major or minor axis; andplacing said flextensional body in an environment where vibrationalenergy is to be detected; and detecting the amplified variationindicative of received vibrational energy with said sensor.

A still further aspect of the present invention provides a method ofmanufacturing a vibration sensing device according to a first aspect ofthe present invention comprising the steps of: (i) mounting blocks on anoutside surface of said flextensional body at either end of said minoror said major axis: (ii) holding said flextensional body within chucksadapted to pass around and hold the outer edges of said blocks within anoptical coil winding apparatus; (iii) passing an optical fibre through areservoir of resin, such that a layer of resin coats said optical fibre,said optical fibre exiting said reservoir of resin via a needle, saidneedle being operable to position said optical fibre above saidflextensional body held within said chucks and being arranged to allow asuitable amount of resin to coat said fibre; (iv) rotating saidflextensional body such that said optical fibre is wound about saidblocks mounted at either end of an axis of said flextensional body toform a coil of optical fibre wound around said axis, said coil beingattached to said blocks by said resin and at least one end of saidoptical fibre being accessible for connection to external opticalcomponents.

The manufacture of these vibration sensing devices involves the directwinding of the coil onto the flextensional body. This is done using anapparatus that has been adapted to hold the flextensional body. Theneedle provides an excellent means for positioning the fibre accuratelyfor mounting on the formers at either end of the flextensional body andfor supplying the correct amount of resin.

Preferably, said step (iv) of winding said optical fibre is performedsuch that coil is wound under tension. Winding the coil under tensionmeans that the flextensional body is pre-stressed. Thus, when itexperiences vibrational forces acting in different directions this doesnot cause both compressional and extensional forces to act upon theellipse. Rather in normal operation the net force acting along thesensing axis of the device remains compressive or extensional and withinthe linear displacement regime. This avoids signal distortion.

In preferred embodiments the method comprises a further step ofcontinuing to rotate said flextensional body after winding said coiluntil said resin has set. This inhibits any drooping of the coil as itsets.

A yet further aspect of the present invention comprises the use of aflextensional body to detect vibrations. The shape function offlextensional bodies provides amplification of some signals and therebyprovides a sensitive way of detecting vibrations.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a plurality of shapes that exhibit a flextensionalproperty;

FIG. 2A shows a schematic side view of the core of a vibration sensingdevice according to an embodiment of the present invention wherein thesensing coil is coupled along the major axis of the device;

FIG. 2B shows the core of FIG. 2A with its dimensions in mm;

FIG. 3 shows a top view of an optical coil wound around the vibrationsensing device of FIG. 1;

FIG. 4 shows a vibration sensing device according to an embodiment ofthe present invention comprising the core illustrated in FIGS. 2A and 2Bmounted in a housing;

FIG. 5 shows a schematic sectional view of the core of a vibrationsensing device according to another embodiment of the present inventionwherein the sensing coil is coupled along the minor axis of the device;

FIG. 6 schematically shows a plurality of vibration sensing devicesarranged in series;

FIG. 7 shows a plurality of optical fibres arranged in parallel and inseries;

FIG. 8 shows three vibration sensing devices according to an embodimentof the present invention mounted orthogonally to each other as avibration sensing package; and

FIG. 9 shows an apparatus used to wet wind the coils.

FIG. 1 shows several examples of shapes whose geometry is such that theyexhibit a flextensional property. Arrow B shows the direction of themajor axis of a cross section of these shapes, while arrow C shows thedirection of the minor axis. The flextensional principle can be definedas the action of a shell, wherein the shell is put into flexure inresponse to an extensional and/or compressional force. If the shellexhibits an extension or compression along one of the major or minoraxis of a cross section of the shell, then this causes flexure whichproduces a movement along the other axis. Depending on which axis theextension or compression occurs along, either the motion or the force isamplified through the mechanical advantage generated by the shapefunction. In the case of a flextensional acoustic transmitter the soundradiating surface of the shell is put into flexure when mechanicallydriven by an extensional vibration of a drive motor. In the presentdevice wherein shapes having this geometry are used as receivers, themethod of operation is reversed. The surfaces which are used to emit theacoustic signal into the surrounding medium are now used to receive thesignal which causes a vibrational extension and/or compression along oneaxis and results in flexing of the body and movement along the otheraxis. IN the case of a receiver the driver motor is replaced with asensor which senses this flexing.

FIG. 2A illustrates the core of a vibration sensing device according toan embodiment of the present invention and shows a flextensionalelliptical shell 11 having end pieces or formers 12. The formers aremounted on the elliptical shell at the ends of the major axis of theshell in the plane containing the major axis normal to the minor axis.An optical fibre sensing coil 13 is wound around the formers andelliptical shell along the major axis of the shell. The formers 12 havecurved edges (see FIG. 3) and thus allow the fibre coil to be woundaround the ellipse without too much strain and associated problems offibre breakage. The formers 12 also provide the optical fibre with alarger radius around which to bend than would the ellipse edge byitself. A larger bend radius of the optical fibre reduces opticallosses. Furthermore, direct winding of the optical coil on theflextensional body would inhibit its deformation and thereby reduce thesensitivity of the device. A mass (shown in FIG. 4 is mounted to theflextensional elliptical shell 11. The mass is between 40 and 100 grams,typically between 60 to 80 grams.

FIG. 2B shows the same core in perspective. As can be seen theflextensional elliptical shell 11 is 22 mm long and 12 mm wide. Theformers 12 are 5 mm long and 11 mm tall. These are preferred dimensionsand other dimensions are also envisaged.

The elliptical shell is preferably formed from metal with the end piecesor formers being made of metal. The material properties of the metal arenot greatly affected by changes in temperature particularly by the rangeof temperatures that are expected to be encountered in seismicapplications and thus such temperature variations should not appreciablyaffect the physical response of the device.

In use, when a component of acceleration occurs parallel to the minoraxis (arrow A), the force required to accelerate the inertial mass(shown in FIG. 4) generates a strain along the minor axis, and a strainof the opposite sense will be set up along the major axis. The use ofthe elliptical shell in this arrangement means that the force that isapplied to the optical fibre sensor (or other sensor in otherembodiments) mounted along the major axis is greater than the force thatwould be applied to the sensor if it was mounted along the minor axis,and this leads to an increase in the device's sensitivity. In otherwords the force is amplified through the mechanical advantage generatedby the shape function of the elliptical shell.

FIG. 4 shows a vibration sensing device according to an embodiment ofthe present invention comprising the core illustrated in FIG. 2, havingan elliptical shell (11), coil (13), outer casing (24), and an inertialmass (21) mounted within the elliptical shell. The elliptical shell hasholes at either end of the minor axis. A bolt (22) passes through one ofthese holes and into the inertial mass (21) connecting the inertial massto one end of the minor axis. The other hole is used to attach the otherend of the minor axis of the shell (11) to the rigid outer casing (24).A washer (23) is used to lift the shell (11) away from the case so thatthe end pieces 12) do not foul on the case (24). In some embodiments,rather than being attached to an outer casing the device is attached viathe hole in the minor axis to the structure whose acceleration is beingsensed.

FIG. 5 illustrates the core of a vibration sensing device according to afurther embodiment of the present invention and shows a flextensionalelliptical shell (11) having end pieces or formers (12). In thisembodiment the formers are mounted on the elliptical shell at the endsof the minor axis of the shell in the plane containing the minor axisnormal to the major axis. An optical fibre sensing coil (13) is woundaround the formers and elliptical shell along the minor axis of theshell. The formers (12) have curved edges and thus allow the fibre coilto be wound around the ellipse without too much strain and associatedproblems with breakage of the fibre. The formers 12 also provide theoptical fibre with a larger radius around which to bend than would theellipse by itself. A larger bend radius of the optical fibre reducesoptical losses.

As in the previous embodiment the elliptical shell is preferably formedfrom metal with the end pieces or formers being made of metal.

In this embodiment, the vibration sensor senses motion in the directionof the major axis of the ellipse (arrow A) by utilising the shapefunction of the geometry of the shell to change the orientation of thevibration and to achieve some mechanical amplification. In effect theforce required to accelerate the inertial mass generates a strain alongthe major axis, and a strain of the opposite sense will be set up alongthe minor axis. The use of the elliptical shell in this arrangementmeans that the displacement that is applied to the optical fibre sensor(or other sensor in other embodiments) mounted along the minor axis isgreater than the displacement that would be applied to the sensor if itwas mounted along the major axis. In other words the displacement hasbeen amplified through the mechanical advantage generated by the shapefunction of the elliptical shell.

In both embodiments vibrational forces or acceleration forces parallelto the minor axis but acting in different directions can cause bothcompressional and extensional forces to act upon the minor axis of theellipse. These forces are in turn transferred as compression or tensionto the fibre. This is undesirable as it may lead to non-linearities inthe response that can cause signal distortion. To avoid distortions ithas been found to be preferable to pre-stress the device so that innormal operation the net force acting along the minor axis of the devicemust remain compressive or extensional. To ensure low distortion whenthe device is excited, it must remain in the linear displacement regime.To this end the optical fibre coils are wound under stress.

Given that individual sensors are sensitive to vibrations parallel to aparticular axis, three sensors mounted orthogonally with each other, sothat vibrations in each of three dimensions can be detected are oftenmounted together as a unit. Further, multiplexing can then occur with aplurality of these units being mounted together in an array. In somedevices a hydrophone is also mounted within the package. This providesinformation on the shear wave.

FIGS. 6 and 7 illustrate possible ways of multiplexing the individualvibration sensing devices to form an array. The multiplexing regimesused are Time Division Multiplexing (TDM) in the array of FIG. 6 andboth Time Division Multiplexing and Dense Wavelength DivisionMultiplexing (DWDM) in the architecture of FIG. 7.

FIG. 6 shows the individual optical sensing coils 63 arranged in series.The coils 63 are bracketed by couplers 61. Attached to one of theoutputs of the coupler 61 is a mirror 62, the other output is connectedto a sensing coil 63. These couplers 61 and mirrors 62 are so arrangedthat there is a reflector before and after each coil so that a pair oflight pulses contained within the fibre are reflected from the mirrors62. The first pulse is split by the first coupler 61 a portion of thepulse is reflected back and the rest is directed into the coil 63, thispulse is then split by the next coupler 61 a portion of the light isthen reflected back from the mirror. At the same time the second pulseis being split by the first coupler 61 and one portion of the light isreflected back. The lengths of the fibres and the timing of the pulsesare selected such that the second pulse returning from the first mirrorand the first pulse returning from the second mirror coincide on thereceiver at the same time. Changes in phase between the two signals canbe used to detect changes in length and/or refractive index of the coilresulting from strains imposed on the coil.

FIG. 7 shows a plurality of these serially connected strings ofvibration sensing devices or seismic sensing devices connected inparallel. The strings of seismic sensing devices are multiplexed andconnected in parallel, using DWDM architecture. Items 71 areMultiplex/Demultiplex units these isolate a single frequency from thesource and direct it to the relevant Seismic sensing device string andthen multiplex the signal back into the signal fibre.

FIG. 8 shows three of the vibration sensing device arranged orthogonallyto each other. These three devices may be mounted in a housing with ahydrophone to form a vibration sensing package. This package can then beused to provide information on both magnitude and direction of anyreceived vibration.

FIG. 9 shows an apparatus used to wet wind the coils 13 around theflextensional body. When manufacturing the vibration sensing deviceaccording to an embodiment of the present invention the basic ellipticalshell is modified to make it suitable for mounting an optical fibre onwithout causing the fibre to break. End or cheek pieces are added toeach side (as shown in FIGS. 2 and 5), these are plates which extend theentire length of the coil and are the same width (or length depending onembodiment) as the elliptical shell. This produces an interesting shapethat needs to be held by chucks in a wet winding machine to wind theoptical fibre coil onto the device. Thus, special holders 43 are made toaccept the ellipse 11 and end pieces 12. These holders comprise a recess44 into which the end pieces 12 fit. This recess serves to both hold theflextensional body and to provide keepers to prevent the fibre escapingduring the winding process. The holder comprises a broad spindle 46 thatserves as a support around which the fly leads from the device can bewrapped during manufacture to keep them safe.

When winding the coil, the flextensional body is mounted and thenclamped between the holders, one of which is mounted onto the tailstockof a winding machine. A length of fibre is initially drawn through thebath and the needle prior to the bath being filled with resin. Thislength is then passed once around the flextensional body 11, 12 andsecured to the winding mandrel on spindle 46. The bath is then filledwith fluid epoxy resin and hardener mixture or other similar bondingmaterial and winding is commenced. The fibre 57 passes through theneedle 55 which is used to position the fibre in the correct positionand to strip the excess resin from the fibre 57. Item 54 holds theneedle rigidly in position relative to the bath 56 and the positioningarm of the precision winding machine.

When 20 m of fibre has been wound onto the flextensional body 11, 12,the winding is stopped. The bath is then emptied and a further fly leadis then drawn through and secured to the spindle 46. To prevent anysagging of the composite coil the device is rotated until the resin hasgelled. Depending on the type of resin used a post cure may be advised.

As the process and arrangement of the coil is such that the fibre isfree at both ends a number of the vibration sensors can be wound in onego. For example, the three components for the three axis measurementscan be wound in one process. This not only reduces the constructiontime, it may also reduce optical losses between devices as it reducesthe need for splicing.

The winding machine has an integral tension controller which adjusts andmaintains the tension applied to the fibre as it is wound. As explainedabove the coil is wound under tension and it is important that arelatively constant tension is applied throughout the coil. Using theneedle 55 positioning system it is not necessary to remove any excessresin as only the correct amount of resin is left on the fibre 57.

Once the epoxy resin has cured then the device can be assembled from thechosen parts. Care must be taken to not damage the fibre 57 as it exitsthe vibration sensing device.

The Optical Fibre 57 used in an embodiment of this invention has a 6micron core with an 80 micron cladding. However, other types of opticalfibre could be used. The epoxy resin used must have a high modulus totransfer the strains from the flextensional body 11, 12 to the fibre 57.

The optical vibration sensing devices of embodiments of the presentinvention are particularly well suited to assembly into an array,individual signals being isolated with the use of time division orwavelength division multiplexing. In particular, the accessibility ofboth ends of the optical fibre in individual vibration sensing devicesaids in this array architecture.

In summary, embodiments of this invention provide a highly sensitiveseismic sensing device which exhibits a low cross-axis sensitivity andcan be easily multiplexed using both time division and dense wavelengthdivision multiplexing.

1. A vibration sensing device comprising: a hollow flextensional bodyhaving a cross section that has a major and a minor axis, saidflextensional body being operable to deform in response to receivedvibrational energy and thereby produce a variation in a predeterminedproperty exhibited by said body in the direction of one of said major orminor axis, said flextensional body being shaped such that saidvariation in said predetermined property is amplified in the directionof the other of said major or minor axis; and a sensor coupled with theflextensional body and operable to detect the amplified variationindicative of received vibrational energy.
 2. A vibration sensing deviceaccording to claim 1, wherein said predetermined property comprisesforce, vibrational energy received along said minor axis causingdeformation of said flextensional body, with a force acting along saidminor axis and an amplified force acting along said major axis, saidsensor being coupled to said flextensional body along said major axis todetect the amplified force.
 3. A vibration sensing device according toclaim 1, wherein said predetermined property comprises displacement,vibrational energy received along said major axis causing displacementof said flextensional body along said major axis and an amplifieddisplacement along said minor axis, said sensor being coupled to saidflextensional body along said minor axis to detect the amplifieddisplacement.
 4. A vibration sensing device according to any of thepreceding claims claim 1, said vibration sensing device furthercomprising an outer housing for housing said vibration sensing device.5. A vibration sensing device according to claim 1, said vibrationsensing device further comprising a mass mounted on said flextensionalbody.
 6. A vibration sensing device according to claim 2, said vibrationsensing device further comprising a mass mounted on said flextensionalbody and wherein said minor axis has a first and a second end, said massbeing mounted to said flextensional body in the proximity of said firstend of said minor axis.
 7. A vibration sensing device according to claim2, said vibration sensing device further comprising an outer housing forhousing said vibration sensing device and a mass mounted on saidflextensional body, wherein said minor axis has a first and a secondend, said mass being mounted to said flextensional body in the proximityof said first end of said minor axis and wherein said flextensional bodyis mounted to said outer housing via mounting means, said mounting meansconnecting a portion of said flextensional body in the proximity of saidsecond end of said minor axis to said housing.
 8. A vibration sensingdevice according to claim 3, said vibration sensing device furthercomprising a mass mounted on said flextensional body and said major axishaving a first and a second end, and said mass being mounted to saidflextensional body in the proximity of said first end of said majoraxis.
 9. A vibration sensing device according to claim 3, said vibrationsensing device further comprising an outer housing for housing saidvibration sensing device and a mass mounted on said flextensional bodyand wherein said major axis has a first and a second end, said massbeing mounted to said flextensional body in the proximity of said firstend of said major axis and wherein said flextensional body is mounted tosaid outer housing via mounting means, said mounting means connecting aportion of said flextensional body in the proximity of said second endof said major axis to said housing.
 10. (canceled)
 11. A vibrationsensing device according to claim 1 wherein said flextensional bodycomprises a tube with an elliptical cross section.
 12. A vibrationsensing device according to claim 1, wherein said mass is mounted withinsaid hollow flextensional body.
 13. A vibration sensing device accordingto claim 1, wherein said flextensional body comprises an outer wall,said outer wall having a substantially uniform thickness. 14-17.(canceled)
 18. A vibration sensing device according to claim 1, whereinsaid vibration sensing device comprises an accelerometer.
 19. Avibration sensing device according to claim 1, wherein said sensorcomprises a strain sensor
 20. A vibration sensing device according toclaim 1, wherein said sensor comprises an optical fibre coupled to saidflextensional body such that deformation of said flextensional bodyproduces a strain in said optical fibre which imposes a variation in atleast one predetermined property of an optical signal transmittedthrough said optical fibre, said optical fibre being arranged such thatat least one end is accessible for optical coupling to an optical devicecomprising a detector for detecting said changes in said at least onepredetermined property of said transmitted optical signal.
 21. Avibration sensing device according to claim 20, wherein said opticalfibre is coupled under stress to said flextensional body.
 22. Avibration sensing device according to claim 20, wherein saidpredetermined property comprises displacement, vibrational energyreceived along said major axis causing displacement of saidflextensional body along said major axis and an amplified displacementalong said minor axis, said sensor being coupled to said flextensionalbody along said minor axis to detect the amplified displacement, saidvibration sensing device comprising blocks mounted on the outer surfaceof said flextensional body at either end of said minor axis, and saidoptical fibre sensor comprising an optical fibre coil, said opticalfibre coil being coupled to said flextensional body by being woundaround said blocks.
 23. A vibration sensing device according to claim 20wherein said predetermined property comprises force, vibrational energyreceived along said minor axis causing deformation of said flextensionalbody, with a force acting along said minor axis and an amplified forceacting along said major axis, said sensor being coupled to saidflextensional body along said major axis to detect the amplified force,and wherein said vibration sensing device comprises blocks mounted onthe outer surface of said flextensioal body at either end of said majoraxis, said optical fibre sensor being coupled to said flextensional bodyby being wound around said blocks.
 24. (canceled)
 25. A vibrationsensing device according to claim 20, wherein said optical fibre iscoupled to said flextensional body such that both ends of said opticalfibre are accessible for optical coupling to further optical devices.26. A vibration sensing package, comprising three vibration sensingdevices according to claim 1, each of said three vibration sensingdevices having a sensor coupled along an axis of said sensing device,said three vibration sensing devices being mounted such that said axesalong which respective sensors are coupled are arranged orthogonally toone another.
 27. A vibration sensing package according to claim 26, saidpackage further comprising a hydrophone.
 28. A vibration sensing system,comprising: a first plurality of vibration sensing devices according toclaim 20; an electromagnetic radiation source and an electromagneticradiation detector; said optical fibres of said first plurality ofvibration sensing devices being arranged in optical communication witheach other and with said electromagnetic radiation source and detector;said electromagnetic radiation source being operable to transmit anoptical signal into said optical fibres of said plurality of vibrationsensing devices; and said electromagnetic radiation detector beingarranged to receive electromagnetic radiation output from said pluralityof vibration sensing devices and to detect a variation in at least onepredetermined property of said output optical signal.
 29. A vibrationsensing system according to claim 28, where said first plurality ofvibration sensing devices are arranged optically in series.
 30. Avibration sensing system according to claim 28, said sensing systemfurther comprising a plurality of partial radiation reflectors, saidplurality of partial radiation reflectors being arranged before andafter each of said plurality of vibration sensing devices; wherein saidelectromagnetic radiation source is operable to transmit a plurality ofpulses into said first plurality of vibration sensing devices such thata pulse of radiation that is reflected back through one vibrationsensing device by a reflector immediately after said vibration sensingdevice reaches said electromagnetic radiation detector at the same timeas, and interacts with, a subsequent pulse reflected by a reflectorimmediately before said one vibration sensing device; said variations insaid at least one predetermined property of said optical signal detectedby said electromagnetic radiation detector being variations in phase.31. A vibration sensing system according to claim 30, further comprisinga signal processor including a time division demultiplexer, said signalprocessor being operable to process signals produced by saidelectromagnetic detector in response to said variations in phase and toisolate signals from individual vibration sensing devices using saidtime division demultiplexer.
 32. A vibration sensing system according toclaim 29, further comprising: a second plurality of vibration sensingdevices arranged optically in series with each other, said secondplurality of vibration sensing devices being arranged optically inparallel with said first plurality of vibration sensing devices; and afirst and second wavelength multiplex/demultiplex unit operable toisolate a single frequency; wherein said electromagnetic source isoperable to produce pulses of radiation at first and second frequenciesand said first and second wavelength multiplex/demultiplex units arearranged such that pulses of said first frequency are transmitted fromsaid source to said first plurality of vibration sensing devices andpulses of said second frequency are transmitted from said source to saidsecond plurality of vibration sensing devices.
 33. A vibration sensingsystem according to claim 32, further comprising at least one furtherplurality of vibration sensing devices and at least one furtherwavelength multiplex/demultiplex unit, said at least one furtherplurality of vibration sensing devices being arranged optically inparallel with said first and said second plurality of vibration sensingdevices; wherein said electromagnetic source is operable to producepulses of radiation at first, second and at least one further frequencyand said at least one further multiplex/demultiplex unit is arrangedsuch that pulses of said at least one further frequency are transmittedfrom said source to said at least one further plurality of vibrationsensing devices.
 34. A vibration sensing system according to claim 31wherein said first plurality of vibration sensing devices are arrangedoptically in parallel.
 35. A vibration sensing system according to claim28, comprising a plurality of vibration sensing packages according toclaim 26 and wherein said sensor comprises an optical fibre coupled tosaid flextensional body such that deformation of said flextensional bodyproduces a strain in said optical fibre which imposes a variation in atleast one predetermined property of an optical signal transmittedthrough said optical fibre, said optical fibre being arranged such thatat least one end is accessible for optical coupling to an optical devicecomprising a detector for detecting said changes in said at least onepredetermined property of said transmitted optical signal.
 36. A methodof detecting vibrations comprising: coupling a sensor to a hollowflextensional body having a cross section that has a major and a minoraxis, said flextensional body being operable to deform in response toreceived vibrational energy and thereby produce a variation in apredetermined property exhibited by said flextensional body in thedirection of one of said major or minor axis, said flextensional bodybeing shaped such that said variation in a predetermined property isamplified in the direction of the other of said major or minor axis; andplacing said flextensional body in an environment where vibrationalenergy is to be detected; and detecting the amplified variationindicative of received vibrational energy with said sensor. 37-51.(canceled)